A projective projector for viewing an image reflected from a screen by setting the projector to the front of a screen and projecting the image to the screen has been used so far as one type of projectors. In this case, it is preferable to set the projector so that the projective direction of the projector becomes vertical to the screen face so that the center of a projected image from the projector and the center of the screen are present on the same horizontal line and projective angles up to the upper and lower ends and right and left ends of the image displayed on the screen from the center of the projected image of the projector become equal to each other. However, because it is not always possible to set the projector to the front of the screen, a keystone corrective function serving as corrective means by image processing and lens shift function serving as corrective means by an optical system are used.
The keystone corrective function is a function for correcting a distortion generated when projecting an image to a screen from a diagonal direction. For example, when projecting an image to a screen from the front, an image area displayed on the screen becomes a rectangle shown in FIG. 5(a). When projecting the image to the screen in a diagonal direction from the downside, the image area becomes the trapezoid whose upper side is longer than the lower side as shown by a continuous line in FIG. 5(b) because of the different between distances up to the screen. When projecting the image to the screen in a diagonal direction from the upside, the area becomes the trapezoid whose lower side is longer than the upper side as shown in FIG. 5(c). When projecting the image to the screen in a diagonal direction from the right side, the area becomes the trapezoid as shown n FIG. 5(d). When projecting the image to the screen in a diagonal direction from the left side, the area becomes the trapezoid shown in FIG. 5(e). When projecting the imager to the screen from directions of right diagonal downside, right diagonal upside, left diagonal downside, and left diagonal upside, the area is displayed as deformed quadrangles such in FIGS. 5(f), 5(g), 5(h), and 5(i).
A function for correcting the distortion of an image is keystone correction. By changing enlargement-contraction rates in horizontal direction and/or vertical direction on a display device such as liquid crystal, performing adjustment so that a distortion on a screen and a distortion of an image area on the display device are canceled, and projecting a distorted image, it is possible to display the image as a rectangular image display area having a normal aspect ratio on the screen. The keystone correction can output a distorted quadrangle by correcting the quadrangle to the original rectangle when angles are kept in a certain range. However, the image quality is slightly deteriorated.
However, the lens shift function is a function for shifting an image display area displayed on a screen in parallel. As shown in FIG. 6, this shifts the position of an image on the screen by adjusting the position of a lens. For example, when assuming that an image area on the screen when the lens position is origin is the rectangular area shown in FIG. 6(A), it is possible to change display areas by parallel shift while keeping the size of the rectangle at the origin as shown in FIGS. 6(B) to 6(E) by adjusting the position of the lens. It is needless to say that adjustment can be made in all directions including not-illustrated vertical and horizontal directions. In the case of the correction by the lens shift function, though the moving area of an image on a screen is restricted because the working range of a lens is small, there is an advantage that the image quality is not deteriorated.
Thus, as correction means when using a projective projector, there are the keystone function serving as correction means by image processing and the lens shift function serving as correction means by an optical system. By properly using these correction means, it is possible to make correction to a rectangle having a normal aspect ratio.
However, when moving the image display area in parallel by the lens shift function in the right diagonal upper direction as shown in FIG. 6(D) and then diagonally projecting an image to a screen from the left diagonal downside, the image display area on the screen in this case is distorted differently from the case in which a lens is present at the origin position. Thus, even if trying to correct the distortion by keystone correction, correction to a rectangle having a normal aspect ratio cannot be made. That is, because the keystone corrective function and lens shift function are respectively independent functions, correction by one function is not reflected on correction by the other function. When correction cannot be made by using the lens shift function, it is necessary to perform keystone correction by once returning the lens up to the origin and projecting an image to the screen in a diagonal direction. Otherwise, correction to a rectangle having a normal aspect ratio cannot be made.
Moreover, when using the both functions, they cannot be used up to correction limits of the both. Therefore, unless they are used in a range narrower than their correctable ranges, correction to a rectangle having a normal aspect ratio cannot be made. That is, though a correctable range (angle) by the lens shift function and a correctable range (angle) by the keystone corrective function are respectively set, correctable ranges do not become the sum of angles even if using the both ranges. In fact, there is a problem that correction to a rectangle having a normal aspect ratio cannot be made by the keystone correction unless the correctable ranges are respectively used in a narrower range.
Because of the above problem, a projector in which the keystone corrective function serving as corrective means by image processing is matched with the lens shift function serving as corrective means by an optical system and correction by lens shift is reflected on keystone correction is desired and Patent Document 1 is already proposed to provide the projector.    [Patent Document 1] Japanese Patent Laid-Open No. 2003-195416